The present invention relates to the wrist device of a robot, and more particularly to a wrist device which has the wrist portion with built-in speed reducers to afford three degrees of rotating freedom orthogonal to one another.
Reducing the size and weight of a wrist portion, reducing the diameter of an arm, and raising the speed and precision of a motion have been required of industrial robots in recent years in order to do jobs such as welding, applying a sealant or assembling components, where an appliance as well as the wrist portion for holding it, and a part of the arm for supporting the wrist portion are inserted into, for example, the car body of an automobile. It is also necessary that the attitude of the appliance can be selected at will at the place of insertion, such that three degrees of rotating freedom are usually required of a wrist device. The wrist device of an industrial robot fulfilling these requirements is disclosed in U.S. Pat. No. 4,548,097 by way of example.
The wrist device of the industrial robot of this type includes a wrist portion which has three degrees of rotating freedom extending orthogonally to one another and intersecting at a single point. Speed reducers having high reduction ratios are assembled in the wrist portion. The rotative power of driving motors mounted on the rear end of an arm is lowered immediately before a driven member through transmission shafts of triple structure penetrating the interior of the arm, gears disposed within the wrist portion, etc., so as to drive and turn the driven member. The wrist device has special features, such that the occupation volume of the wrist portion within a job domain is small owing to the setup of the degrees of freedom stated above. Moreover, since the speed reducers are arranged at the final stage of a power transmission system, the degradation of a positioning accuracy and the interference of operations among the degrees of freedom attributed to the backlashes of the gears, etc. can be suppressed to the utmost, and the power transmission system from the driving motors to the input shafts of the speed reducers can be made small in size and light in weight.
In this wrist device, construction the power is transmitted to the wrist portion by the triple coaxial differential mechanism of great axial length penetrating through the arm. In the case of putting the shafts into the triple structure, the minimum inside diameter of the shaft which is arranged at the outer periphery of any of the shafts is determined by the outside diameter of the shaft to pass therein or the dimensions of bearings for rotatingly supporting the shafts at the ends of both these shafts. For this reason, especially the bores of the intermediate and outermost transmission shafts become larger than are necessary. Consequently, the weight increases and is augmented by the inertia of shafting. Such unnecessarily large bores apply also to one set of bevel gears which are mounted on the end of the intermediate shaft in the wrist portion so as to perform a directional change. Moreover, in this wrist device, the innermost shaft and the intermediate shaft transmit the power to the speed reducers built in the wrist portion, and they rotate fast at a speed near the output revolutions of the driving motors. Particularly in the driving motor for driving the intermediate transmission shaft, an unnecessarily large amount of power is unfavorably consumed for rotating the transmission elements at high speed. In addition, precise shaft machining is required and also the assemblage and maintenance become complicated for the reasons that the shafts are long and are supported through the bearings at both the ends, they are rotated at high speed and they have the triple structure. Besides, the arm needs to be lengthened more in a robot which coats, for example, the inner part of the car body of a large-sized automobile with a sealant. In this case, the disadvantages mentioned above become more conspicuous, and especially the shafts rotating at high speed might cause bending distortions due to their own weights and thereby induce excessive vibrations. It is therefore subject to a limitation that the arm is lengthened with its outside diameter held constant.